Electric fuse



March 22, 1938. H. c. HOBAN 2,111,628

ELECTRIC FUSE Filed Jan. 15-, 1937 s Sheets-Sheet 1 A Fzg. 3.

II I Q 7103 31097 2 32 Flg. 4. d

nw z-wra? HUGH CHARLES 'HOBAN March 22, 1938. H; c. HOBAN 2,111,628

ELECTRIC FUSE Filed Jan. 15, 1937 5 Sheets-Sheet 3 ig. we

FUSING CURRENT AMPERES l l l I 1 2o 40 60 80 I00 I20 I40 I60 m0 200 TEME-MINUTES- M/VHWOR HUGH CHARLES HOBRN Patented Mar. 22; 1938 2,1 1

ELECTRIC FUSE Hugh Charles Hoban, Dartford, England, assign=- or to W. T. Henleys Telegraph Works Company Limited, London, England, a British company Application January 15, 1937;" Serial No. 120,748 In Great Britain January 21, 1936 8 Claims. (01. 200-131) This invention relates to electric fuses which comprise a fuse element in the form of a wire or strip which makes connection at each end to a terminal member and is ruptured by the passage of an excessive current. To ensure that rupture takes place only at a suitable point, generally at a point approximately midway between its ends, it is usual to make the cross sectional area of the part at which it is desired rupture shall take Both these constructional features, individually and jointly result in the central portion running at a higher temperature than it would otherwise do. ilonsequently the element fuses at a current which more nearly approaches its working current.

The improved construction of fuse element is not to be confused with elements of the well known bimetallic type in which there is present lo place less than that of the remaining parts, genan insertion of a metal or alloy having a relal erally the end or stem portions of the element. iively low melting p 'fl- Such a sertion is so This has the effect of increasing the resistance p opo t o ed e at ve to the adjacent Parts o e of that portion so that, under working condielement that it will melt in the event of the sustions, it assumes a higher temperature than the tained overload conditions. Nor is the new con- 13 end portions with the result that the working struction to be confused with elements of the 15 current of the element more nearly approaches pe in which a ce t a us e port on is conthe fusing current. This is an advantage since nected to stem portions of larger current carrythe fuse will blow at a smaller overload current ing capacity by means of an extremely thin film than in the case of an element of uniform section. of soldering metal. In the present case the in- It is an object of the present invention to prosertions between the central and stem portions 20 vide a fuse element of the type described in which are made of substantial and predetermined thickthe ultimate fusing current of the element may ess or/ e p pos of efieeting app e rebe made to approach Still more closely to the duction in the transmission of heat from the cenworking current or, in other words, to improve tral portion, in which fusion takes place, to the the fusing factor of a fuse element, the fusing stem portions, and they are not intended to, and factor being equal to the ultimate fusing current are so proportioned that they do not, under any divided by the working current. circuit conditions with which they are designed to with this object in vie we provide a fuse elecope, melt and break the junctions between the ment comprising a central fusible portion and centre and stem p rts of t e e ment before fustem portions, in which the central portion is o of the ce t al po t on occurs. 30 electrically connected to, and spaced apart to a he construc n of t e junc on between the predetermined extent from, each stem portion by central and stem parts of the element is po an insertion of substantial thickness of a metal tent. If the mass of the insertion m tal or ey or alloy having a relatively high thermal resistis unduly large the thermal resistance instead of ance, whereby the conduction of heat from the being increased y even be deeieasedn the fusible central portion to the adjacent portions of ther hand if it is too small the effect of the inhigh current carrying capacity is reduced. Sertion will be negli y satisfactm'y With the same object in view we provide a fuse sults have been obtained by overlapping the cenelement which comprises a central fusible portral portion of wire for a distance along the stem 40 tion of wire or narrow Strip a d Stem por io of equal to about three times the diameter of the 40 relatively Wide Strip. at least D of each Stem wire and uniting the overlapping portions by an P01111011 being in the form of expanded metal insertion having a thickness of, approximately, that is to say, in the form of sheet metal cut and one. fifth to one seventh the length of the over bent or stretched to form a lattice. Preferably Ian V the are formed mm a stnp of The invention will be further described with 45 metal sht lengthwlse at puniber of interthe aid of the accompanying drawings which mediate its ends and having he me a on one or Show, by way of example fuse elements com both sides of each slit pulled or pressed out of the h d normal p1 ane (or other Surface) of the strip to structed 1n accordance with t e 1nven ion an a form a lattice. Such a strip has a considerably cartndge type of fuse fitredlwlth number of 50 higher superficial area than has a plain strip of suck} elemefltsthe awmgs the same cross sectional area It is th f re in Figure 1 is a side elevation of one form of fuse more intimate contact with the surrounding coolelement. Figure 1A is a fragmentary e io ing medium and, consequently, facilitates the partly in section and on an enlarged scale, of the as transfer of heat from the stem to the medium. elements shown in Figure 1,

Figure 2 is a plan view of the element shown in Figure 1,

Figure 3 is a cross-sectional view, taken on the line IlI-III of Figure 1, of the element shown therein.

Figure 3A is a cross-sectional view on an enlarged scale, taken on the line IIIA-IIIA of Figure 1A, of the element shown therein,

Figure 4 is a perspective view of a second form of fuse element,

Figure 5 is a side elevation of the element shown in Figure 4,

Figure 6 is a plan view of the element shown in Figures 4 and 5,

Figure 7 is a cross-sectional view, taken on the line VII-VII of Figure 5, of the element shown therein,

Figure 8 is a cross sectional elevation of one half of a cartridge fuse fitted with elements of the form shown in Figures 4, 5, 6, and 7, and

Figure 9 is an end elevation of the fuse shown in Figure 8.

Figure 10 is a graph of the relationship between fusing current and time which demonstrates the advantage yielded by the improved construction of element of the form shown in Figures 1, 2, and 3.

Referring first of all to Figures 1 to 7 inclusive it will be seen that the element comprises a central portion I' in the form of a wire of circular cross section and stem portions 2 in the form of thin strips of metal of greater current carrying capacity. The central portion overlaps the stem portions for a distance along the stem equal to approximately three times the diameter of the wire and these overlapping parts are united by soldering them together in such a way that each overlapping part of the wire is spaced apart from the strip by the solder 3, a distance approximately equal to half the diameter of the wire. A convenient way of ensuring correct spacing is to form a small upstanding projection on the extremity of the strip, as shown at l in Figure 1. The width of the insertion between the overlapping parts is determined by the diameter of the wire, the soldering metal being allowed to form a natural meniscus between the lateral surface of the wire and the adjacent fiuxed surface of the strip. It is preferred to form both the central and stem portions of silver. In this case suitable materials for the thermal resistance insertions are pure lead solder, and a solder consisting of an alloy of lead containing a small percentage of tin.

In both examples of construction the central fusible portion is shown as a wire of circular cross-section. This form is advantageous in that it reduces the area of the radiating surface of the central portion to a minimum but it will be obvious that central fusible portions of other form, for instance, wire of noncircular crosssection or narrow metal strip, may Just as readily be used if desired.

In the example of construction shown in Figures 1, 2, and 3, the stem portions 2 are in the form of thin fiat strips, but in Figures 4 to '7 inelusive the stem portions 2 are of a modified form tive to the strip and is preferably parallel with the edge of the strip, as shown. The slits of each group are shown to be in line with the slits of the other group but this is not essential. It will be seen that these slits 3 and l divide two short lengths of the stem into three parts. At each of these places the two outer parts (I or I) are displaced in one direction out of the plane of the strip and the intervening part (9 or III) is displaced in the opposite direction. The outer parts I at one place are preferably displaced in the opposite direction to the outer parts I at the adjacent place, as is clearly shown in Figure 4. Naturally, the number of groups and the number of slits per group may vary and may be more or less than in the example shown in the drawings.

In Figure 8 is shown one half of a cartridge fuse employing a number of elements similar to that described with reference to Figures 4, 5, 8, and '7; the other half of the fuse, being exactly similar, has been omitted from the drawings. The fuse comprises a barrel ii of ceramic material provided with a number of passages l2 extending from end to end thereof. A metal cap I! is positioned on each end of the barrel II and is secured thereto by means of a cemented joint I. This cap is provided with apertures corresponding to the passages II. In each e I! is disposed a pair of elements it. The central parts I of these elements are bunched together and located in the centre of the passage but the stems 2 diverge from one another so that near the end of the passage they lie on opposite sides thereof. The extremities of the stems pass through the apertures in the cap It and are bent over as shown at It and held in contact with the outer face of the cap by soldering or welding and/or by means of an outer cap I! which fits over the inner cap and carries the contact blades l8. The ends of the passages I! are plugged with asbestos plugs l9 and an asbestos pad is inserted between the bent over ends of the elements I! and the outer metal cap H, in order to reduce the transmission of heat from the interior of the fuse to the contact blades.

The elements shown in, and described with reference to, the drawings each have only a single central fusible portion, but naturally each element may be fitted with two or more such portions in cases where it is considered desirable.

The invention is particularly advantageous when applied to fuse elements of cartridge fuses where the element is embedded in a material having a thermal conductivity that is low compared with that of the metal of the element. Normally the major part of the heat generated in the element will be conveyed to the terminals of the fuse. By the improved method of uniting the stem and central portions, this transmission of heat may be reduced and the heat generated be dammed in the centre part of the element with the result that the fusing factor is considerably decreased. This is clearly shown by the graphs in Figure 10 in which Curve A shows the relationship between fusing current and the time taken to fuse for a normally constructed cartridge fuse nominally rated at 300 amperes and Curve B the relationship for a cartridge fuse with an element of similar dimensions and rating but having the central and stem portions united in accordance with the invention, the stems in both cases being of the form shown in Figure 1. It

will be seen that whilst the normally constructed fuse requires approximately 95 minutes tofuse at a current of twice its nominal rating, the improved fuse will blow in less than 12 minutes under the same overload conditions.

In order to demonstrate the greater heat dissipating properties possessed by the improved construction of stem, tests were undertaken on two t pes of elements both formed of strip metal of precisely the same dimensions. In both types the width of the stem portions of the element was 0.5 inches and the thickness 0.05 inches and the width of the central fusible portion was 0.2 inches and the thickness 0.006 inches. In one case the stems were of plain strip and in the other case the stems were constructed precisely as shown in-Figure 4. Acartridge fuse fitted with a pair of such elements with plain stems fused with a current of 114 amperes in 17 minutes 41 seconds. A similar cartridge fuse fitted with a pair of such elements having stems constructed precisely as shown in Figure 4 fused with the same current in 22 minutes 33 seconds. In the former case. with an ambient temperature of 20 C., the temperature of the fuse cap was '71.5 0., that of the fuse contact 64.2" C. and that of the circuit contact 51.5 C. -In' the latter case, with the same ambient temperature, the temperatures at corresponding placeswere considerably lower, being 62.6'' C. at the fuse cap, 585 C.

at the fuse contact and 47.6 C. at the circuit contact, thus demonstrating that the element having stems constructed in accordance with the invention dissipated considerably more heat into the are quenching medium in which the element was embedded. It will be appreciated that this reduction in temperature of the circuit contacts is particularly advantageous in cases where parts of these contactsare embedded in insulating compounds as is the case with fuse disconnecting network boxes and the like.

What I claim as myinvention and desire to secure by Letters Patent is;-

1. A fuse element for an electric fuse, com-.

prising a centralportion in which fusion isinitiated, stem portions and means serving to connect each end of said central portion electrically to, and space it apart to a predetermined extent from, the, adjacent stem portion, each of said means being an insertionof substantial thickness of a metallic material having a relatively high thermal resistance.

2. A fuse element for an electric fuse, comprising a central portion of silver injwhich fusion is initiated, stem portions of silver and means for electrically connecting said stem poix tions to the ends of said central portion, each of said meanscomprising an insertion of sub-' stantial thickness of metallic material having a thermal resistance considerably higher than that of silver. r

3. A fuse element for-an electric fuse, comthermal resistance.

4. A fuse element for an electric fuse, comprising a central fusible portion of wire orv narrow strip and stem portions of relatively wide strip, at least part of each stem portion being in the form of expanded metal.

5. A fuse element for an electric fuse comprising a central fusible portion of wire or narrow strip and stem portions, said stem portions each consisting of relatively wide metal strip having, near the central portion of the element, a plurality of groups of parallel slits extending longitudinally of the strip and having the metal on both sides of each slit displaced from the normal surface of the strip to form a lattice.

electrically connecting the ends of said fuse element to saidterminal caps, and a cooling medium, in which the stems of said fuse element are embedded, contained in said tubular member.

'7. A fuse element for an electric fuse, comprising a central portion of wire or strip in which fusion is initiated, stem portions which are of relatively wide strip and comprise parts in the form of expanded metal, and means serving to connect said central portion electrically to and space it to a pre-determined extent from, each of said stem portions, each of said means-being an insertion of substantial thickness of a metallic material having a relatively high thermal resistance.

8. A fuse element for an electric fuse, including a central portion in which, under any circuit conditions with which the fuse is designed to cope, fusion is initiated, a pair of stem portions, and a pair of heat barriers, consisting of metallic material of which the. thermal resistance is relatively high compared with that of the central and stem portions, one of said barriers being inserted between, and connecting together, one end of said central portion and one of said stem portions and the other of said barriers being inserted between, and connecting together, the other end of said central portion andthe other of said stemportions.

noon CHARLES HOEAN. 

